I have a fair amount of experience using Stackexchange (mostly using Math.SE). One common concern for users is when a question or answer is downvoted. It is considered good practice to always provide a comment when you downvote a question or an answer. That way the poster will be able to rectify whatever might be the problem. This practice is, however, usually not enforced in any way by moderators.
I suggest that we all make it a habit to always provide a comment (unless sufficient comments already exist) when downvoting anything.